is DEX useless now?

Unfortunately, Big Bang was the final nail in the coffin of the Knights of Cygnus.
Before that, a level 120 KoC could participate in at least some "end game" content (such as Maple has). Now, they can't even hit the more important bosses.

I disagree. Cygnus is an awesome feature. Level one to 120 and you will get a decent storyline, a killer boost to your main char and the ability to start a new adventurer directly at lvl 50 (tho it will be some months before it'll be implemented). Not to mention that it is actually fun to level it. What else could you ask out of it?